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Force acting on a permanent magnet

  1. Mar 13, 2009 #1

    A wire carrying a current I has a magnetic field from a permanent magnet crossing it at right angles. The force on the wire is equal to the cross product of Current Length with the Magnetic Field, B.

    What is the force experienced by the magnet? and how do we know it?
    Please do not invoke Newton's 3rd Law in the answer.
    Last edited: Mar 13, 2009
  2. jcsd
  3. Mar 14, 2009 #2
    Without using Newton's 3rd law you would need to use the magnetic field of a current carrying wire and treat the permanent magnet as a sheet of current. The answer, if the problem were to be tractable, would be identical to the one given by Newton's law.
  4. Mar 14, 2009 #3
    I guess what I am really asking is does any one know of any experiment that was ever performed that measured the force acting on the magnet and where I can find it. If there is then I don't have to perform it myself.

    Incidentally I think that part about treating the permanent magnet as a sheet of current is incorrect as you would have to model it as an amp turn.
  5. Mar 14, 2009 #4
    Hi, why don't u try googling ur reqd. topic........ you might find several interesting links! Gud Luck :smile:
  6. Mar 14, 2009 #5


    Staff: Mentor

    You do realize that a large fraction of the world's electric motors work on this principle, right? I guess anyone rolling up their power window or flipping their power locks would be considered such an experiment.
  7. Mar 14, 2009 #6
    Yes I've searched, as far as I know this experiment has never been done before and written about. My preferred browser is Yahoo incidentally. No the fact electric motors work does not tell me what I want to know. I'll give it some more time and see if anyone knows about such an experiment.
  8. Mar 14, 2009 #7
    Experiments of this kind were done in the 19th century, go to your local library and look for a compilation of Faraday's notebooks.

    The term 'amp turn' is not standard in physics, perhaps it is an engineering term that I am unfamiliar with. The hits I get on google related to this term are from crackpots building perpetual motors. In any case, of course it is appropriate to describe a permanent magnet in terms of currents, since all magnetic phenomena arise from and have to do with moving charges cf. Maxwell's equations.
  9. Mar 14, 2009 #8


    Staff: Mentor

    Why not? You want the force exerted on a permanent magnet by a current carrying wire, right? That's a motor.
    Last edited: Mar 14, 2009
  10. Mar 14, 2009 #9
    First we use Faraday's induction equation:
    where V = voltage and dA = area, n =unit vector perpindular to dA

    V = - d/dt integral (B(t) dot n dA) over A

    ["dot" means dot product]

    Magnitude of B is defined by the above equation, using defined values of volts, area, and time.

    1 tesla defined as 1 volt-second per square meter as per above induction equation.

    Then we calculate the force given by the Lorentz equation F = I x B
    Last edited: Mar 14, 2009
  11. Mar 15, 2009 #10
    Thanks Bob, yur reply was actually too technical as I couldn't figure out what it had to do with my problem. My own qualatative analysis is that if you modeled the magnet as a loop carrying a current and crossed it with a wire carrying a current at any arc you want, since the contour Integral of the forces acting on the wire are holomorphic, yields a net force on the wire, but there is no net force on the loop as all forces cancel.

    I guess the easiest thing to do now is just design and run the experiment and write about it.

  12. Mar 15, 2009 #11
    Last edited: Mar 15, 2009
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